Add preferred image aspect for ImageBlock

[tidyui]

When working with images as blocks in the main content, add a setting for "Preferred Aspect" where the editor can choose how the image show be displayed in the current context. The available aspects should be:

• Original
• Landscape
• Portrait
• Square

For the developer, add a method on the MediaHelper that resizes an ImageBlock with a given width according to the currently selected preferred aspect.